Conventionally, as a hydrometallurgy method for recovering a valuable metal such as nickel and cobalt from a nickel oxide ore having a low nickel content represented by a limonite ore and the like, a high temperature pressure acid leaching (HPAL: High Pressure Acid Leaching) method using sulfuric acid has been known.
For example, in a high pressure acid leach method to obtain a nickel-cobalt mixed sulfide, as shown in FIG. 4, a pretreatment step (1), a leaching step (2), a solid-liquid separation step (3), a neutralization step (4), a dezincification step (5), a sulfurization step (6), and a detoxification step (7) are included (for example, see Patent Document 1).
In a pretreatment step (1), a nickel oxide ore is ground and classified to obtain a slurry.
In a leaching step (2), sulfuric acid is added into the slurry obtained in the pretreatment step (1), the resultant mixture is stirred at a temperature of 220 to 280° C., and high temperature pressure acid leaching is performed to obtain a leach slurry.
In a solid-liquid separation step (3), a leach slurry obtained in the leaching step (2) is subjected to solid-liquid separation to obtain a leachate containing nickel and cobalt (hereinafter, referred to as “crude nickel sulfate aqueous solution”), and leach residues.
In a neutralization step (4), a crude nickel sulfate aqueous solution obtained in the solid-liquid separation step (3) is neutralized.
In a dezincification step (5), hydrogen sulfide gas is added into the crude nickel sulfate aqueous solution neutralized in the neutralization step (4), and zinc is precipitated and removed as a zinc sulfide.
In a sulfurization step (6), hydrogen sulfide gas is added into the dezincification final solution obtained in the dezincification step (5), and a nickel-cobalt complex sulfide and a nickel barren solution are obtained. In a detoxification step (7), a heavy metal is solidified and removed as a hydroxide by a final neutralization treatment, and a leach residue generated in the solid-liquid separation step (3) and a nickel barren solution generated in the sulfurization step (6) are detoxified.
In general, many kinds of heavy metals are contained in a nickel oxide ore, the nickel oxide ore is dissolved by using sulfuric acid under high temperature high pressure conditions, and then a chemical treatment is performed to remove impurities, subsequently, a required metal such as nickel is recovered in a sulfurization step. In a sulfurization reaction, hydrogen sulfide gas, and a salt such as sodium hydrogen sulfide, and sodium sulfide are used, however, an unreacted sulfide remains after the sulfurization step.
In a case where hydrogen sulfide gas is used in a sulfurization reaction, unreacted hydrogen sulfide gas is dissolved in a solution after the reaction, and also in a case where a salt such as sodium hydrogen sulfide, and sodium sulfide are used, hydrogen sulfide gas may be generated depending on the state of the solution. The solution after sulfurization is reused or subjected to a wastewater treatment in a step (for example, see Patent Document 2).
When hydrogen sulfide gas is generated in these steps, it is not desired from the viewpoint of the working hygiene and the environment.
Conventionally, as a removal method of dissolved hydrogen sulfide in a solution (barren solution) after sulfurization reaction, a method in which a nickel oxide ore slurry after leaching is added into a barren solution after sulfurization, a reduction action with trivalent iron existed in a large amount in the nickel oxide ore slurry after leaching is utilized, and hydrogen sulfide is oxidized to sulfur to decrease the concentration of the dissolved hydrogen sulfide has been known.
However, in a case where a method described above is used, the addition of acid and the slurry after leaching are repeatedly required, and further, dissolved hydrogen sulfide still exists in a solution after reaction, therefore, further decrease of the dissolved hydrogen sulfide is required.